Over the years, many adhesive compositions have been developed for bonding various materials or objects such as elastomeric substrates and metal surfaces. As bonding technology continues to develop, adhesive compositions are being exposed to increasingly demanding bonding conditions while the range of ingredients utilized in adhesive compositions is continually being circumscribed by environmental regulations. In order to fare well in this somewhat dilemmatic atmosphere, future adhesive compositions will need to utilize environmentally acceptable components which result in adhesive materials capable of withstanding relatively severe bonding conditions.
One example of increasing severity of bonding conditions can be found in the increasingly popular injection mold bonding process. Injection mold bonding is effective since production can be increased due to the use of higher cure temperatures and shorter cure times and since the rubber/metal bonding process can be automated. However, injection molding processes are carried out at high temperatures (320.degree.-350.degree. F.) which can interfere with the bonding effectiveness of the adhesive composition utilized in the process.
Examples of previously developed adhesive composition can be found in U.S. Pat. Nos. 3,258,388 and 3,258,389. These rubber-to-metal adhesive compositions are described as utilizing a nitroso aromatic compound as a type of crosslinker which enhances adhesion to the rubber substrate. While these adhesive compositions are effective for bonding rubber to metal under many types of conditions, it has been found that the aromatic nitroso compound tends to sublime at the higher temperatures currently experienced in injection molding processes. This sublimation can cause the incoming rubber to react with the aromatic nitroso compound and to be cured before it has a chance to fill the injection mold cavities and react with the adhesive on the metal surface. The pre-cured rubber will not mix with the rest of the rubber in the mold, resulting in the formation of weak boundary layers that can lead to premature failure in the cured article. These adhesives, as well as many other adhesive systems, are also somewhat disadvantageous in that they typically require the application of two coats of material, such as a primer and an overcoat, in order to obtain sufficient adhesion.
An attempt to develop an adhesive composition free of aromatic nitroso compounds is disclosed in U.S. Pat. No. 3,640,941. This adhesive composition utilizes a graft polymer of polybutadiene and substituted cyclopentadiene monomer, dibasic lead phosphite, resorcinol, and a solvent. While this composition avoids the use of aromatic nitroso compounds, it requires the utilization of a lead-containing compound which can be detrimental to the environment and hazardous to the health of persons handling the adhesive composition.
A one-coat adhesive composition is needed that will utilize nontoxic, environmentally acceptable components as well as withstand increasingly demanding bonding conditions such as those experienced during injection molding processes. Such an adhesive composition should also produce an adhesive bond that will resist exposure to adverse environments such as high temperature fluids and corrosive agents.